Just as an older car might need a higher grade of gas, the aging human body needs a higher grade of fuel, in the form of food, to keep running smoothly.

"Mother Nature is not kind to us," says registered dietitian Joan Salge Blake, author of Nutrition & You. "As we age, the metabolic rate declines, so we need fewer calories. But at the same time we need as many or even more nutrients. So, we have to be smart about diet choices. We have to make sure each bite counts."

Besides the metabolic slowdown, there are a host of other changes – loss of muscle and bone mass, hormonal declines, immune system malfunction, digestive problems, and more – that all alter nutritional needs. To help seniors make smart decisions, scientists with the U.S. Department of Agriculture's Human Nutrition Research Center on Aging (HNRCA) at Tufts University have created the MyPlate for Older Adults infographic, an update of the old food pyramid specifically designed for seniors.

Half of MyPlate for Older Adults is filled with fruits and vegetables, with the rest evenly split between whole grains, and protein and dairy. Healthy fats and oils, and herbs and spices, are included on the side. The takeaway is to eat more whole foods and less of the processed stuff.

"In the crowding-out method, we add more healthy foods to a client's meal plan so there is less room for the unhealthy foods," explains Greenfield, a geriatric nurse practitioner who works with seniors in the Dementia Prevention Program at Florida Atlantic University's Comprehensive Center for Brain Health. "You are literally crowding out the junk until you reach a well-balanced diet that is sustainable."

Here are some of the special nutritional needs of seniors and the foods that best supply them:

Antioxidants

Why they are needed: Flavonoids, phytochemicals, polyphenols, and other antioxidants found in fruits and vegetables are some of the most potent anti-aging compounds around. They help protect cells from renegade ions called free radicals, which cause oxidative stress and the resulting inflammation that contributes to heart disease, cancer, Alzheimer's, arthritis, and other chronic problems. Mitochondria, the little organelles that power cells, are particularly susceptible to oxidative stress.

"That's why older people tend to feel tired so frequently," says Dr. Sandra Kaufmann, author of "The Kaufmann Protocol: Why We Age and How to Stop It."

What to eat: HCRCA scientists, Greenfield and many others suggest choosing produce that has the deepest colors, which signals high antioxidant content. Compounds such as the orange-colored carotenoids found in carrots and sweet potatoes; red lycopene in tomatoes and beets; and green lutein in kale and spinach; are all great free radical fighters.

"I tell people, when they look at their plate, they want to see a rainbow," Greenfield said. "The different colors mean you are getting a wide range of protective phytochemicals.

Tip: Do not shy away from frozen produce. "Frozen vegetables and fruits are equivalent or superior to fresh when it comes to nutrients because they are harvested and preserved at their peak," says Alice Lichtenstein, Director of the HNRCA's Cardiovascular Nutrition Laboratory.

Fiber

Why it is needed: As we age, the digestive process slows down, contributing to constipation, gastric reflux, and other problems. Fiber, which is basically the parts of food we cannot digest, sweeps things along the GI tract like an intestinal broom. It also feeds the good bacteria in our microbiome and helps to optimize cholesterol and sugar levels in the blood.

What to eat: Much of the fiber is removed in processed foods. Even though it is plentiful in the natural world, Americans typically do not get near enough. Good sources are fruits, vegetables, whole grains, legumes, nut, and seeds. Avocados and artichokes are two of most fiber dense products of the vegetable kingdom, but legumes have even more. Good choices are beans, lentils, chickpeas, and split peas. Whole grains also have significant amounts of fiber. Steel-cut oatmeal is a great source along with barley, bran, brown rice, wheat, and rye.

Tip: Soluble fiber needs to be hydrated to work effectively, but as people age, their sense of thirst tends to diminish, causing less fluid intake. Try to drink plenty of water and unsweetened beverages to keep things flowing smoothly in the gut. It also helps to eat water-laden foods, including apricots, grapes, grapefruit, and, of course, watermelon.

Protein

Why it is needed: Hormonal changes and other factors cause people to start losing muscle mass after the age of 30, but it can really accelerate following middle age. Also called age-related sarcopenia, the loss can cause frailty, falls, and a more sedentary lifestyle, which only exacerbates the problem. While resistance exercise, such as weight training, is the top treatment to minimize muscle loss, protein provides the necessary amino acids to build muscle.

What to eat: All whole foods have some amount of protein, but animal products have a complete form. The problem in the U.S. is it often comes in red or processed meats, which also contain unhealthy saturated fats. Better choices are fatty fish such as wild salmon, sardines, and mackerel. They not only contain protein but also heathy fatty acids that protect against heart disease and Alzheimer's. Shellfish is also a good protein source and high in zinc, a trace mineral seniors often lack that helps with immunity. Eggs are great in moderation, and low- or no-fat dairy products have complete protein. There are also plenty of good partial protein sources in non-meats, including legumes, nuts, and seeds.

Tip: Try the grain quinoa, which has all nine essential amino acids. "Quinoa is one of nature's few complete plant proteins and is also high in B vitamins, vitamin E, several minerals and antioxidants," Greenfield noted.

Calcium

Why it is needed: Bone loss is a normal part of aging but can help be prevented or delayed with exercise and consuming calcium-rich foods. Supplements are popular, but they can cause trouble if the calcium, which is hard for the body to absorb, winds up as deposits in joints, blood vessels, and other unwanted places.

What to eat: Blake suggests no- and low-fat dairy products as good calcium sources, in part because they are fortified with vitamin D, which the body needs to absorb calcium from food. Other excellent sources include plain yogurt, cabbage, broccoli, spinach, lima beans, black beans, almonds, kale, and mustard greens.

Tip: Calcium absorption is dependent on vitamin D, and vitamin K plays an important role in keeping the mineral in the bones and out of arteries. Since Americans are often deficient in these two nutrients, Blake and other experts suggest taking Vitamin D and K supplements.

Potassium

Why it is needed: High blood pressure increases naturally with age, and a bad ratio between sodium and potassium can make it worse. Recommendations call for about three times as much potassium as sodium in the diet, but in the U.S., those numbers are often reversed.

What to eat: Bananas seem to get a lot of attention as potassium sources, but plain old potatoes with their skins have twice the content. In fact, bananas fall behind several other potassium-rich foods, including avocados, lima beans, sweet potatoes with skin, prunes, Brussels sprouts, and cantaloupe.

Tip: Beets can also help lower blood pressure because they are loaded with natural nitrates, which turn into nitric oxide gas during the digestive process. The gas relaxes blood vessels, allowing for better blood flow.

Probiotics

Why they are needed: Gut flora in the microbiome drops dramatically with age, and that can have a dramatic effect digestion, immunity and even brain health.

What to eat: Fermented foods such as plain yogurt with live cultures, sauerkraut, kimchi, tempeh, miso, kefir, and pickles all help to repopulate the microbiome with "good" bacteria.

Tip: Instead of downing a cold beer or frozen margarita on a hot day, try a chilled glass of Kombuchi, a slightly alcoholic fermented tea.

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